Operating conditions sometimes impose reliability and maintenance requirements that force you into selecting specific equipment. One prime example is level measurement in high temperature, high fouling services. While non-contact devices often are the best choices to cope with fouling, high temperatures may make typical radar or ultrasonic units impossible to use. Other non-contact measurement techniques, such as gamma ray and neutron backscatter, may cost too much for your budget. However, there’re a couple of less-expensive options: purged differential pressure (DP) cells and bubblers.
A standard level-measurement system uses a DP cell attached to the vessel at two points: one in the vapor space above the liquid and the second in the liquid at its lower level position; Figure 1 shows this connection to a collector tray in a distillation tower. The usefulness of the level signal depends upon the accuracy, precision and reliability of the cell. If the liquid is prone to plugging, then the impulse lines from the DP cell to the vessel may clog, causing inaccurate readings.
Adding purges to either or both taps (Figure 1), as required, can keep the lines clean. Typical installations use restriction orifices (RO) in the purge lines to prevent excessive flows. Even so, the pressure drop from the purge connection point to the vessel can cause incorrect level readings —falsely high if the pressure drop is on the liquid side and falsely low if the pressure drop is on the gas side.